1. It will be slightly worse than the 5dIII sensor, to keep the differentiation. The DR will be slightly less, the high ISO will be 1/2-1 stop worse, and the video will have more moire/aliasing.
well compared to sony/nikon they already lack behind (*).
there is no price to win in making a even "worser" sensor.
and what you wrote about (moire, DR, high iso) why not using the 5D Mk2 sensor then?
the 5D MK2 is exactly as you describe it.
no need to develop a new sensor, only to make it a bit worse then the 5D MK3 sensor.
(*) and trust me i can tell from my customers that most of them buying a 2000-3000$ camera are well aware of that fact. but then im not a online retailer....
2. There are still plenty of consumers who can't differentiate between spec sheets and rely on MP. So, now Canon has the logical upgrade path of 18mp for their APS-C, 20MP for their entry full-frame, and 22MP for their 5dIII. And the 1DX is differentiated by being $6k+
again... from my experience the 2000-3000$ customers are not such noobs anymore.